Compose Preview — CI setup

Wiring the compose-preview CI surface into a repository: a compose-preview/main baselines branch, sticky before/after diff comments on pull requests, and the choices that decide whether one workflow is enough or you need two.

This skill is about standing the pipeline up and keeping it working. For reading the diffs it produces — reviewing a UI PR, triaging a flaky preview — see the compose-preview-review skill. For driving the renderer by hand, see compose-preview.

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